Sam, Sahh and Map gene expression during cell division and paralytic shellfish toxin production of Alexandrium catenella (Dinophyceae)

The expression of genes potentially involved in paralytic shellfish toxin (PST) biosynthesis was quantified during cell division of the dinoflagellate Alexandrium catenella. Basic precursors of the neurotoxic PST molecule(s), such as S-adenosylmethionine (SAM), have been hypothesised, but no genes coding for PST production have been definitively identified in dinoflagellates. Sam encodes the enzyme SAM synthetase and two other candidate PST genes S-adenosylhomocysteine hydrolase (Sahh) and methionine aminopeptidase (Map) have been previously identified in the closely related PST producing dinoflagellate Alexandrium fundyense. PST production and expression of Sam, Sahh and Map were examined concurrently during cell division in A. catenella. The PST profile of A. catenella was dominated by C2 (30-37 mol%), GTX5 (12-19 mol%) and GTX6 (27-33 mol%) and was constant over the cell cycle. However, the net cellular PST production rate varied over the cell cycle and was highest (127 fmol cell -1 h-1) during the G2+M phase of the cell cycle (56% cells in G2+M), slowing considerably (-14 fmol cell -1 h-1) as cells entered the G1 phase (66% cells in G1). Quantitative real time PCR revealed that Sam, Sahh and Map were up-regulated (doubling in each case) during PST production and the G2+M phase of the cell division cycle before irradiance. Alexandrium catenella actively produced PSTs during this dark period, suggesting that photon irradiance was not a direct trigger for PST production. Interactions between cell division and PST production appear complex and the present study has demonstrated correlations between PST production and expression of Sam, Sahh and Map, which warrants further investigation.